This invention relates to improvements in steering column assemblies. It also relates to a method of assembling a steering column assembly.
Steering columns for vehicles, such as cars, are increasingly required to have the ability to change length. With the prevalence of airbags, it is important that the wheel is not placed too close to a driver nor too far away. Allowing the length to be adjusted allows the driver to set the correct “reach” of the steering column. The requirement for a change in length is also important in a crash, as the column should be able to collapse under impact to reduce the forces on the driver's chest if it strikes the steering wheel.
One of the simplest ways to achieve the required length adjustment is to provide a two-part column that comprises an inner tube and an outer tube with the inner tube being able to slide within the outer in the manner of a traditional telescope. These assemblies are known as tube-in-tube steering columns. A typical tube in tube column assembly is shown in FIG. 1 of the accompanying drawings; whilst this shows part of an electric power assisted steering system, the invention itself is not so limited. The inner and outer tubes define a shroud which sits around, supports and protects a collapsible steering shaft that is connected to the steering wheel of the vehicle.
Where a column assembly of the kind shown in FIG. 1 is adjustable for reach and or rake (up and down movement) the outer tube must be held rigidly by a releasable clamp mechanism to a column support bracket that is fixed to the vehicle body structure. Ideally, the clamp system would lock the outer tube in place and simultaneously cause the outer tube to grip the inner tube. The benefits of this would be: (a) to maximise the bending stiffness of the total column assembly in the clamped position and (b) to ensure there is a predictable amount of sliding friction between the inner and outer tubes, in the clamped condition, which can contribute a specified proportion of the total energy-absorbing crash force required in a crash, and (c) in cases where reach adjustment is to be provided, to allow the inner and outer tube to freely slide in an unclamped condition.
Clamping a continuous cylindrical outer tube onto an inner tube requires a high clamp force (a tube naturally does not easily compress without unwanted buckling), so to reduce the force needed it is known to provide an elongate slot in the outer tube. A pair of flanges is provided, one on each side of the slot. The flanges can be forced together by a suitable clamp mechanism when in the clamped position to at least partially close the edges of the slot together. A clamp mechanism that includes a clamp pin that passes through the flanges has been proposed to provide this load to the flanges.
The applicant has appreciated that a potential problem with such a steering column assembly is that when unclamped the amount of free play between the inner and outer tubes is hard to control. The inner diameter of the outer tube is not always consistent between samples, deformation often occurring during machining of the slot into the tube. Past attempts to compensate for this have involved complex arrangements of lock nuts on each end of the clamp pin used to hold the flanges together. The clamp mechanism itself when unclamped can therefore control how far the flanges can be allowed to spread apart and provide some pre-load to the flanges. Whilst these designs can be effective, they do require careful set up and can be easily knocked out from the optimal set up over time.